First paragraph: Platinum (as well as many other metals) may
be prepared in finely divided forms that are
porous at all levels of magnification. Cracks and
crevices in the sponge-like opaque matrix then
allow visible light (of wavelength around 0.5 μm)
to enter, whereupon it undergoes multiple reflections
and is unlikely to be reflected back out. This
effectively produces a high value for optical
absorption, resulting in a velvety black appearance
to the eye, even though the platinum is in
the metallic state. Other porous structures (such
as snow and plaster) consist of inherently transparent
or translucent crystals, so light can be
refracted as well as reflected out and they appear
matt white.